Golf ball compositions are formed from a variety of materials, including balata and ionomer resins. Balata is a natural or synthetic trans-polyisoprene rubber. Balata covered balls are favored by the more highly skilled golfers because the softness of the cover allows the player to achieve higher spin rates sufficient to more precisely control ball direction and distance, particularly on shorter shots.
However, balata covered balls are easily damaged, and thus lack the durability required by the average golfer. Accordingly, alternative cover compositions have been developed in an attempt to provide balls with spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance.
Ionomer resins have, to a large extent, replaced balata as a cover stock material. Chemically, ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically-unsaturated carboxylic acid having 10-90% of the carboxylic acid groups neutralized by a metal ion. See U.S. Pat. No. 3,264,272. Commercially available ionomer resins include, for example, copolymers of ethylene and methacrylic or acrylic acid neutralized with metal salts. These are sold by E.I. DuPont de Nemours and Co. under the trademark “SURLYN®” and by the Exxon Corporation under the trademark “ESCOR®” and the trademark “IOTEK®.” These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
U.S. Pat. No. 3,454,280, U.S. Pat. No. 3,819,768, U.S. Pat. No. 4,323,247, U.S. Pat. No. 4,526,375, U.S. Pat. No. 4,884,814, and U.S. Pat. No. 4,911,451 all relate to the use of SURLYN®-type compositions in golf ball covers. However, while SURLYN® covered golf balls as described in the preceding patents possess virtually cutproof covers, they have inferior spin and feel properties as compared to balata covered balls.
Polyurethanes and polyureas have also been recognized as useful materials for golf ball covers since as early as about 1960. For example, U.S. Pat. No. 3,147,324 is directed to a method of making a golf ball having a polyurethane cover, which is durable while maintaining the “feel” of a balata ball. The polyurethane covers can be formed from polyurethane prepolymers cured with curing agents having at least one active hydrogen groups (such as amines and/or polyols), wherein the prepolymers are formed from the reaction of polyols with polyisocyanates.
There are other examples of the use of polyurethane and polyurea as golf ball cover materials. For example, U.S. Pat. No. 4,123,061 discloses that a golf ball can be made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a diamine. U.S. Pat. No. 5,334,673 discloses the use of thermoset and thermoplastic polyurethanes for forming golf ball covers, and in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent and/or a difunctional glycol. U.S. Pat. No. 5,484,870 discloses that golf ball covers may be prepared from polyurea compositions that are prepared by combining an organic isocyanate having at least two isocyanate functional groups with an organic amine curing agent.
Polyurethane is a product of a reaction between a polyurethane prepolymer and a polyol curing agent. The polyurethane prepolymer is a product formed by a reaction between a polyol and a diisocyanate. The curing agent is either a polyamine or a polyol. It has been discovered that a polyurethane prepolymer cured with a slow-reacting curing agent selected from the group of slow reacting polyamine curing agents or polyfunctional glycols produces a golf ball cover that has good durability and performance. These golf balls have been found to have improved shear resistance and cut resistance compared to golf balls having covers made from either balata or soft SURLYN® material.
The first commercially successful polyurethane covered golf ball was Titleist's PROFESSIONAL™ golf ball in 1993. The principal reason for the delay in bringing polyurethane composition golf ball covers on the market was that it was a daunting engineering task to apply a covering of polyurethane composition to a golf ball core to form a golf ball cover having a uniform thickness. Further progress has been made in the area of polyurethane balls, including improved water resistance, UV light stability, abrasion resistance and durability.
There has been further progress in the uses of polyurethane and polyurea, including the use of polyurea/polyurethane hybrids. According to the Polyurea Development Association, a polyurea is defined as the result of a chemical reaction between an isocyanate and an amine. Polyurethane is defined as the result of a chemical reaction between an isocyanate and a polyol. Polyurea/polyurethane hybrid formulations are defined as the result of a chemical reaction between an isocyanate and a mixture of polyol and polyamine reactants. These formulations generally provide an “intermediate” polyurea that displays many of the same properties of a polyurea. The isocyanate can be aromatic or aliphatic in nature and can be monomer, polymer, or any variant reaction of isocyanates, quasi-prepolymer or a prepolymer. The prepolymer or quasi-prepolymer can be made of an amine-terminated polymer resin or a hydroxyl-terminated polymer resin.
Further improvements to golf ball performance is desired in such characteristics as initial velocity of the golf ball. Initial velocity is related to the coefficient of restitution of a golf ball. The interaction between the club head and the golf ball determines the distance and direction the golf ball will travel. The relationship between the speed of the club head and the initial velocity of the golf ball depends on the coefficient of restitution of the golf ball, which varies between different types of balls. The distance that the golf ball travels is dependent on the speed of the club head and the initial velocity of the golf ball, which varies between different types of balls. When the golf ball is struck by the club, it is deformed and flattened by the force of impact. A golf ball with a harder core will deform less than a softer golf ball. In general, a harder ball will travel further than a softer ball because it deforms less and will efficiently transfer more energy from club to ball. During the impact between the golf ball and club head, kinetic energy is transferred and stored as the ball tries to regain its original shape. To obtain maximum distance in the drive, a golf ball must be selected that maximizes restitution for the club speed. Further details may be found at: ffden-2.phys.uaf.edu/211_fall2002.web.dir/josh_fritts/index.html.
Despite these efforts, there is still a need for golf balls with an improved initial velocity without negatively impacting the other desirable properties of golf balls. The present invention is directed to these, as well as other, important needs.